Kevin,
First thought, you may want to check your hardness and dissolved solids. GH should be 1 - 3 degrees and KH should be about the same. A low KH (below 3 dgrees) has little buffering ability and could crash PH. TDS should be about 300 ppm.
Second thought, would be that behavior occurred soon after a water change. A marked increase or decrease in PH or temperature would affect them.
Third thought, were you watching them in your underwear?